To study the effect of plasminogen activator inhibitors (PAI) on fibrinolysis it is essential to be able to specifically measure these proteins in plasma. To this end PAI-1 was purified from cortisol- stimulated HT 1080 fibrosarcoma cells and antisera raised in rabbits. The immunologic relationship of the purified inhibitor to PAI-1 in plasma and platelet extracts was established by immunoblotting and regular and reverse fibrin zymography. Furthermore, the purified product could be immunoprecipitated with antibodies to human or bovine endothelial cell-derived PAI-1. A radioimmunoassay was developed that measures both free and tissue-type plasminogen activator (t-PA)-bound PAI-1 in plasma and has an effective range of 8 to 250 ng/mL. PAI-1 antigen levels showed a twofold increase after 20 minutes of venous occlusion, partially due to hemoconcentration. Approximately one quarter of PAI-1 before and after venous occlusion is derived from platelets. After correction for hemoconcentration and the contribution of platelets to plasma PAI-1 levels, a still significant increase in PAI-1 levels was noted during venous occlusion, which suggests that the local vascular bed releases PAI-1. Concomitant with PAI-1, t-PA antigen levels increased eightfold and fibrinolytic activity 18-fold after 20 minutes of venous occlusion. PAI-1 and t-PA levels tend to augment with age: in a group of older healthy volunteers (mean age, 53 years) PAI-1 levels were twice and t-PA levels 1.7 times higher than those in a group with a mean age of 29 years. Determination of PAI-1 antigen levels before and after platelet aggregation demonstrated that 85% of PAI-1 in platelet-rich plasma is associated with platelets. The average amount of PAI-1 per platelet was 0.3 fg/platelet, ie, 4,000 molecules per platelet.